Inside the new iPhone 6s and iPhone 6s Plus is an upgraded M9 motion coprocessor that borrows new fitness capabilities from the Apple Watch, by estimating your walking or running pace.

Unlike in previous iPhones, where the M-series chip was separate from the main CPU, Apple has integrated the M9 processor into the A9. The M9 is found in the forthcoming iPhone 6s, iPhone 6s Plus, and the iPad Pro.

This new integration into the main CPU has allowed Apple to let the motion sensor to run constantly without draining the device's battery life.

In a first for the iPhone series, the iPhone 6s will measure a user's walking or running pace when they exercise, without the need for battery draining GPS. Previously, this functionality was limited to the S1 chip in the Apple Watch.

The M9 also allows for Siri to be always on. That means users will be able to use the device's "Hey Siri" voice command at all times.

In older devices, "Hey Siri" only works if the iPhone or iPad is plugged into a power source. AppleInsider's own sources were first to reveal earlier this week that the iPhone 6s would be Apple's first device to support always-on voice commands.

Like with previous M-series chips, the M9 also tracks steps, distance, and elevation changes. The M9 measures this data through a number of sensors in the iPhone, including an accelerometer, compass, gyroscope, and barometer.